Seismic velocity changes concentrated at the shallow structure as inferred from correlation analyses of ambient noise during volcano deformation at Izu-Oshima, Japan

We investigate the stress sensitivity of velocity changes at Izu-Oshima in Japan using seismic interferometry method. We calculate cross correlation functions (CCFs) of ambient noises recorded by four seismic stations on the active volcano from 1 January 2012 to 31 December 2015 at 0.5–1 Hz, 1–2 Hz, and 2–4 Hz. Applying moving time windows to calculate cross spectrum between daily CCFs and reference CCF, which is the average for the all observation period, we compute daily velocity changes. The obtained velocity changes vary from −1% to 3% with a dominant period of about 1 year. We calculate areal strains by using Global Navigation Satellite Systems data at the volcano. It is evident that the velocity changes are well correlated with the areal strain changes whose linear trend is removed. The stress sensitivity of velocity changes, which are obtained from the observed velocity changes and areal strains, is lower at 0.5–1 Hz than that at higher frequencies: (7.1 ± 1.3) × 10⁻⁸ Pa⁻¹ at 0.5–1 Hz, (1.4 ± 0.1) × 10⁻⁷ Pa⁻¹ at 1–2 Hz, and (1.3 ± 0.1) × 10⁻⁷ Pa⁻¹ at 2–4 Hz. Modeling the velocity changes of Rayleigh wave propagating in layered structures, we find that the observed velocity changes are concentrated in the upper 1 km of the structure. Since similar frequency dependency is recognized among the stress sensitivity of velocity changes reported in previous studies, we conclude that the velocity changes are localized in the shallow depth because of the increase of confining pressure.